According to Shannon, the maximum possible bit rate C over a radio transmission link depends on the signal-to-noise ratio (SNR) as follows:C=Log2(1+S/N)The highest possible SNR is further limited by the Error Vector Magnitude (EVM) which is a measure of how far away an actual complex transmission symbol is from its ideal location in the complex constellation. FIG. 1 for example shows an ideal complex symbol vector V and the error vector E. The EVM equals E/V. The EVM may be caused by a number of sources such as filter delay, insertion loss, radio channel variations, and clipping of the transmitted signal.
FIG. 2 shows the amplitude of a signal with multiple signal peaks that exceed positive and negative amplitude threshold values. To accommodate these peaks within the DC-voltage range, the gain or mean power level must be reduced. These peaks increase peak-to-average power ratio. A high peak-to-average power ratio results in a low mean power that results in reduced efficiencies for a radio transmitter's power amplifier. For example, a high peak-to-average ratio means less clipping probability, a better EVM, but less mean output power such that path loss increases. A power amplifier with a greater linear range is required. In addition, a larger maximum power requires more current, more cooling, and larger transistors. Thus, a high peak-to-average power ratio results in higher cost due to these cooling and transistor requirements.
To reduce a high peak-to-average power ratio, radio transmitters may “clip” the signal peaks in order to limit the maximum amplitude of the transmitted signal. Clipping thus facilitates higher mean output power which is advantageous on especially longer distances. Unfortunately, clipping introduces a significant amount of in-band noise as well as out-of-band spurious emissions resulting in increased EVM, which in turns means, for example, a lower possible maximum bit rate.
One way to avoid clipping and decrease EVM is to “backoff” or reduce the input power of the power amplifier. FIG. 3 graphs the output power (in mW) to input power (in mW) relationship of a typical power amplifier over time. If the input power is below a maximum power level, then the power amplifier operates in a linear manner where an increase in input power is matched by a proportional increase in the output power, as shown in the linear region. But if the input power exceeds a certain value, then the power amplifier operates in a nonlinear manner where an increase in the input power is not matched by a proportional increase in the output power. The output power is less than ideal in the power amplifier's nonlinear operating range resulting in increased EVM. Moreover, if the mean power value is too high, some peaks are clipped—a source of further non-linearity—as shown in FIG. 3.
When a radio base station transmitter employs code division multiple access (CDMA), the transmitted signal is a composite signal that includes multiple signals directed to multiple radio terminals, each radio terminal signal being encoded with random codes or sequences. When multiple CDMA signals intended for various radio terminals are input to a single power amplifier and amplified, the amplified composite output signal includes peaks in the composite signal are generated that are not typically present in other communication signals where a power amplifier is only amplifying one signal at a time.
FIG. 4 illustrates a multi-signal power amplifier (PA) that receives multiple input signals (e.g., five input signals A-E are shown in FIG. 4) and amplifies them during the same time period. As mentioned above, a multi-signal amplifier may be useful for example in CDMA transmissions where each input signal is coded using a different CDMA code. But multi-signal amplifiers are not limited to CDMA applications or to transmitting to multiple radios. For example, orthogonal frequency division multiplexing (OFDM) transmission may also use a single power amplifier to amplify multiple signals, i.e., sub-carriers, at the same time. And multiple signals might be transmitted to a single radio terminal. The problem is thus related to amplifying multiple input signals during the same time period by a single power amplifier rather than a particular type of transmission or a particular number of receivers.
FIGS. 5A and 5B illustrate the signal peaking problem associated with multi-signal power amplifier using the power amplifier (PA) shown in FIG. 4. FIG. 5A illustrates simplified waveforms for each of the five input signals A-E over the course of eight time slots. FIG. 5B shows the resulting composite output signal generated by the power amplifier (PA). The input signals are superpositioned in the power amplifier (PA) so that the composite signal has a peak in time slot 5 that exceeds a maximum allowed power, and thus, must be clipped.
The inventors realized that existing transmitters make no distinction between first signals provided to a multi-signal power amplifier in which clipping is necessary and other second signals provided during the same time period to the multi-signal power amplifier in which clipping is not necessary. Instead, all of those signals would be clipped resulting unfortunately in increased EVM and other disadvantages described above for the first signals.